Prior to setting forth the background of the invention, it may be helpful to set forth definitions of certain terms that will be used hereinafter.
The term “structured light” as used herein is defined as the process of projecting a known pattern of pixels (e.g. grids or horizontal bars) on to a scene. The projected patterned light is deformed when striking surfaces and analyzing the deformation allows vision systems to calculate the depth and surface information of the objects in the scene. For providing further details of how structured light can be implemented, by way of illustration only, WIPO publication number WO2013088442 is incorporated herein by reference in its entirety.
The term “spatial coding” as used herein is defined as a fixed pattern that is projected onto the scene and is imaged by the sensor's camera. The fixed pattern is designed in such a way that along epipolar lines each region of the pattern can be uniquely identify by considering the local neighborhood pattern. This method is efficient in terms of projected pattern power use and acquisition time, but requires dedication of several pixels to each label and hence results in lower spatial resolution. The main disadvantage is that a fixed pattern lacks the flexibility to respond to different scene conditions by using a different pattern. FIG. 1A illustrates a fixed light pattern 100A exhibiting broken lines according to the prior art. The fixed pattern may be implemented using a mask of phase shifters configured to result in combination of constructive and destructive interferences.
The term “temporal coding” as used herein is defined as illuminating the scene by a series of patterns. The patterns series are designed in such a way that each pattern position in a particular epipolar line is coded by a unique time series. The method can be accurate and has flexibility, but is not power efficient and requires a long acquisition time. FIG. 1B illustrates a temporal light pattern 100B according to the prior art, exhibiting a unique vertical line 111, 112, 113, and 114 for each of time stamps t1, t2, t3, and t4 respectively.
While the ability to dynamically change the pattern is highly desired, the power inefficiency it ensued makes it very unattractive in many applications. In order to allow a dynamic pattern, typically an image source is used. Such an image source either scans the scene progressively or illuminates it simultaneously, with the image forming light source modulated to produce the light and dark areas of the pattern. Considering the maximal output of the light source, its modulation reduces the “on” time and hence reduces the total output power.
Taking, as an example, a constant wave (CW) laser diode as the light source projected using a scanning mirror system. Such a laser diode may emit a certain amount of maximal optical power. When the light is modulated in order to produce the desired pattern, the average intensity is reduced by the modulation and hence a loss of brightness results. This tradeoff is true for any standard use of image projection method.